Set up a new account in Thunderbird for the new server name.

Thanks. I think I can see how to set up a new account with the same email address. I'll want it to be the default account and also to bring across all the data (eg local folders, history) from the existing account. Is this automatic/possible?

I'll want it to be the default account

Once the account with the new server settings has been created just set it as the default one.

I'll want ... also to bring across all the data (eg local folders, history) from the existing account. Is this automatic/possible?

What is the account type of the account with the existing server settings - POP or IMAP? If POP - did you leave all messages on the server?

What account type will be the new account?

Thanks. IMAC before and after

IMAC before and after

I suppose this means IMAP?

With IMAP all mail is managed on the server, so everything will be there once you created the account with the new server settings. Once you confirmed this you can remove the account with the old server settings.

Sorry, typo. should be IMAP

There are lots of "local" historic emails in C:\Users\Roger\AppData\Roaming\Thunderbird\Profiles\ ... , (surprisingly none in ...AppData\Local\...) . I've saved both the Profiles folders. I'm sure the server holds only a few messages generated in the last few days when TB has been down.

I'm feeling encouraged to go ahead with creating a new account unless there's a good reason not to.

There are lots of "local" historic emails in C:\Users\Roger\AppData\Roaming\Thunderbird\Profiles\ ...

I don't what point you're trying to make with "local". If this is an IMAP account all your mail is managed on the server. What you do see in Thunderbird for this account is basically a mirror of what's on the server.

(surprisingly none in ...AppData\Local\...) .

Not surprisingly. This isn't your profile folder.

I'm sure the server holds only a few messages generated in the last few days when TB has been down.

You can log in via webmail to check what's actually on the server. If there are 'only a few messages generated in the last few days', then you must have either deleted anything else, or deliberately moved older messages off of the server to your 'Local Folders' account.

Finally got it working! I've manged to set up a new TB profile using the correct server name and ports and connect to the server OK That was the object of the exercise. I'm left with the old system the new system each under their own profile folders. I manged to copy (by File Explorer) my local archive system to the new profile simply by copying "Local Folders" from the old to the new. The paradox is that this "Local Folders" folder is located under a folder called "Roaming" further up the chain when there is a parallel folder called "Local" which sounds the more natural place for it. The basic TB folders (inbox, draft, sent etc) are mirrored on the server and the local system but when I move an email from eg local inbox to its local archive folder it is automatically deleted from both local and server inboxes. I copied my address books from the old to the new system but ended up with 2 Personal Address Books and 2 collected addresses, one of each from the old and new systems. I'd like to merge them but they seem to work fine so I can live with that. Many thanks for the help. The advice is valuable but reassurance that help is available if needed is worth even more. Much appreciated.

I've manged to set up a new TB profile

I don't know why you did this, it would not have been needed. You can set up multiple accounts in the same profile.

I manged to copy (by File Explorer) my local archive system to the new profile simply by copying "Local Folders" from the old to the new.

If you had kept the same profile, this step would not have been needed.

The paradox is that this "Local Folders" folder is located under a folder called "Roaming" further up the chain

No paradox. "Local Folders" is in your profile, and the profile resides underneath "Roaming". It's always been like this.

when there is a parallel folder called "Local" which sounds the more natural place for it.

There is no profile underneath "Local". It's primarily the cache folder residing underneath "Local".

The basic TB folders (inbox, draft, sent etc) are mirrored on the server and the local system but when I move an email from eg local inbox to its local archive folder it is automatically deleted from both local and server inboxes.

This is how IMAP works.

I copied my address books from the old to the new system but ended up with 2 Personal Address Books and 2 collected addresses, one of each from the old and new systems. I'd like to merge them

You can drag' n drop contacts between address books.

Thanks for this follow up. I may have done this the hard way but am relieved to have got it done. Thanks for your additional comments and suggestions.